The present invention relates to a capacitive pressure sensor and its manufacturing method and, more particularly, to a capacitive pressure sensor in which a sensor portion and an amplifier are formed on a semiconductor monocrystalline layer.
In one of this type of the prior sensors, an amplifier is formed on a main surface of a semiconductor substrate. A diaphragm is formed with a depression on the surface opposite to the main surface. A connection layer electrically connects the diaphragm and the amplifier. A glass plate with an electrode plate is placed, in opposition to the diaphragm, on the main surface of the semiconductor substrate to form a capacitor. This sensor is disclosed in IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. ED-27, No. 5, May 1980, pp 927 to 930. In the capacitive pressure sensor, a pn junction electrically isolates a connection layer for connecting the diaphragm and the electrode plate to the amplifier from its adjacent layer. A large capacitance is formed at the pn junction, in addition to the capacitance formed between a diaphragm and the plate electrode. The capacitor at the pn junction is susceptable to a change in ambient temperature. The prior sensor has the depression in which the electrode plate is vapor-deposited on the glass plate. A dimensional requirement for the depth of the depression is stringent because it determines a capacitance of the sensor. Nevertheless, the accuracy of finishing is not satisfactory. Since the layer for forming the amplifier and the diaphragm deformable by pressure are closely disposed in the semiconductor substrate and since a process for forming the diaphragm is different from a process for forming the amplifier, the yield rate cannot be increased. Further, the formation of them makes it difficult to control the thickness of the diaphragm which is desired to be of 10 .mu.m order in the manufacturing stage. Improvement of a sensitivity of the sensor is also hindered. Another physical feature that the amplifier and the diaphragm are formed on the same side, limits its areal reduction.
A capacitive pressure sensor with no semiconductor substrate is disclosed in Japanese Patent Application Kokai (Laid-Open) No. 143422/80 (corresponding to U.S. patent application Ser. No. 30,587). In this type of the capacitive pressure sensor, a pair of diaphragms oppositely disposed with a gap therebetween are bonded together by frit glass set around the periphery. A capacitor is formed by electrodes vapor-deposited on the inner surfaces of the confronting diaphragms. In manufacturing the capacitive pressure sensors, difficulty encountered in making the expansion coefficient of the frit glass coincident with that of the material of the diaphragm such as ceramics or rock crystal. In the bonding process, it is desired that the melting point of the glass is as low as possible. Nevertheless, it is difficult to find glass whose the melting point is low and whose the thermal expansion coefficient is substantially equal to that of material of the diaphragm, since the expansion coefficient is inversely proportional to the melting point. This causes the above difficulty. The existing difference of the expansion coefficients produces a thermal distortion between the diaphragms and the glass in the heating and cooling processes of the manufacturing the pressure sensors. The thermal distortion leads to a variation of capacitances of the resultant products, or capacitive pressure sensors. The measure tentatively taken for the thermal distortion problem is to use thick diaphragms with increased rigidity. This measure impairs a sensitivity of the pressure sensor, however.